Detection and monitoring of leaked CO2 through sediment, water column and atmosphere in a sub-seabed CCS experiment

Abstract Carbon capture and storage in sub-seabed geological formations (sub-seabed CCS) is currently being studied as a realistic option to mitigate the accumulation of anthropogenic CO 2 in the atmosphere. In implementing sub-seabed CCS, detecting and monitoring the impact of the sequestered CO 2 on the ocean environment is highly important. The first controlled CO 2 release experiment, Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage (QICS), took place in Ardmucknish Bay, Oban, in May–September 2012. We applied the in situ pH/ p CO 2 sensor to the QICS experiment for detection and monitoring of leaked CO 2 , and carried out several observations. The cabled real-time sensor was deployed close to the CO 2 leakage (bubbling) area, and the fluctuations of in situ pH and pCO 2 above the seafloor were monitored in a land-based container. The long-term sensor was placed on seafloor in three different observation zones. The sediment pH sensor was inserted into the sediment at a depth of 50 cm beneath the seafloor near the CO 2 leakage area. Wide-area mapping surveys of pH and pCO 2 in water column around the CO 2 leakage area were carried out by using an autonomous underwater vehicle (AUV) installed with sensors. Atmospheric CO 2 above the leakage area was observed by using a CO 2 analyzer that was attached to the bow of ship of 50 cm above the sea-surface. The behavior of the leaked CO 2 is highly dependent on the tidal periodicity (low tide or high tide) during the CO 2 gas release period. At low tide, the pH in sediment and overlying seawater decreased due to strong eruption of CO 2 gas bubbles, and the CO 2 ascended to sea-surface quickly with a little dissolution to seawater and dispersed into the atmosphere. On the other hand, the CO 2 bubbles release was lower at high tide due to higher water pressure, and slight low pH seawater and high atmospheric CO 2 were detected. After stopping CO 2 gas injection, no remarkable variations of pH in sediment and overlying water column were observed for three months.